Tell me what is Google's Quality Score and what role does it play in ranking?

Submitted by: Muhammad
This used to be a technical question I'd use in the first face-to-face interview but I was stunned to find that some candidates either
a) Did not know what QS was or
b) Thought I was talking about PageRank!

While I do not expect Google's textbook answer, I am hoping the candidate understands the basic concept behind QS (clickthrough rate + other factors) and that QS is part of the Ad Rank formula where:

Ad Rank = CPC bid X Quality Score.

I now use this question to open the technical section of a phone interview. A primary function in their role as PPC analyst will be to educate non-search people. Their ability to explain Quality Score and Ad Rank to me in plain English is something I pay very close attention to.
Submitted by: Muhammad

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